There seems to be a lack of information in your question: You've not told us where you're starting from or where in Florida you wish to fly to!
Possibly the most popular destination in Florida is Orlando. According to the Orlando Airport website the only direct scheduled flights out of the UK are from Gatwick (with BA and Virgin), Manchester (Virgin), Bristol (Continental) and Edinburgh (Continental).
The second most popoular destination in florida is probably Miami. The Miami Airport website shows the only direct flights from the UK operating from Heathrow (BA and American Airlines).
I'm sorry that this doesn't directly answer your question but I hope that it will point you in the right direction.
http://www.virgin-atlantic.com/en/us/index.jsp
http://www.continental.com/
http://www.aa.com/
Chris
PS: It's always been standard airline practice to charge child fares only from 2 to 12 years. The budget European airlines (such as Ryanair) have adopted of an 'under-16' policy but this it's unlikely that you'll find this applied elsewhere.